Isomerization 5-ring dehydrocyclization

When the reactions of alkane molecules larger than the butanes or neopentane are studied, and in particular when the molecule is large enough to form a Cs or a Ce ring, the complexity of the reaction pathway is considerably increased and an important feature is the occurrence, in addition to isomerization product, of important amounts of cyclic reaction products, particularly methylcyclopentane, formed by dehydrocycliza-tion this suggests the existence of adsorbed cyclic species. The question is whether the reaction paths for dehydrocyclization and isomerization are related. There is convincing evidence that they are. Skeletal interconversions involving n-hexane, 2- and 3-methylpentane may be represented. 

Methyl groups, as hydrocarbon surface species, vibrational spectra, 42 214—219 Methylheptane, ring closure, 25 154 3-Methylhexane dehydrocyclization, 30 13 isomerization, 30 7, 14, 39-40 Methylhexane, ring closure, 25 155 Methyl hydroperoxide, catalytic decomposition, 35 161... 

Apparently, nonacidic chromia-alumina catalyzes C5-dehydrocyclization only when the new bond is formed between a primary carbon atom and the aromatic ring. We know that the isomerization of alkylbenzenes over nonacidic chromia-alumina involves free-radical intermediates and proceeds by phenyl or vinyl migration (41, 42). One can speculate that dehydrocycli-zation also has a free-radical mechanism here. 

In the high temperature region consecutive reactions play a role also when the overall conversion is kept low (under 10%) a fraction of isomerization products are formed through a ring opening of the previously produced methylcyclopentane (when hexane is the feed). This indicate that each shortening of the apparent contact time T enhances dehydrocyclization. Contact time T can be varied in a controlled manner by changing the amount of the catalyst and the extent feed. 

The cyclic mechanism (Scheme 7), which involves dehydrocyclization to an adsorbed cyclopentane intermediate C, followed by ring cleavage and desorption of the products, and is responsible for the isomerization of larger molecules on dispersed platinum-alumina catalysts (52, 55). 

Metallocarbene formation by hydrogen shift explains the observed selectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/AljOj (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydro-cylization to gaseous cyclic molecules was determined. Relative rates of 3 2 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-10%) and the temperature (32O°-38O°C). 

Since 1-5 ring closure provides a route for the skeletal isomerization of alkanes, isomerization of substituted benzenes by a cyclic mechanism should also be possible. That was verified by Shephard and Rooney (95), who found that, on 0.5% Pt/Al2O3, interconversion of o-ethyltoluene and n-propyl-benzene accompanied dehydrocyclization to indane (Scheme 80). In these... 

Fig. 16. Relative activities obtained on a Pt-Re-S/Al203 catalyst, coked in a commercial reactor (A) n-C5 reactions, P = 0.5 MPa T = 773 K, (1) isomerization, (2) hydrocracking, (3) dehydrocyclization, (4) hydrogenolysis (to Cl) (B) CH reactions, P = 0.5 MPA, (5) ring contraction, (6) CH dehydrogenation (reprinted from Appl. Catal. 52, 249-262, C. A. Querini, N. S. Figoli, and J. M. Parera,, Copyright 1989, with permission from the publisher, Elsevier Science).

Cyclization—The conversion of aliphatic hydrocarbons containing six or more carbon atoms in a chain to the corresponding aromatic hydrocarbon is known as dehydrocyclization. The reaction sequence is believed to involve dehydrogenation of a paraffin to an olefin followed by ring closure and subsequent dehydrogenation of the ring compound to an aromatic. In many cases, isomerization (see 18) reactions also take place. 

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